Surface light source device

Abstract

A surface light source device comprises a light emitting section (11) made of a single spot light source, a light guide plate (12), a reflective surface (13) provided on the back side of the light guide plate, and a prism pattern (15). On the light emitting surface side of the light guide plate (12), there is provided a directional light scattering film (14) consisting of at least two phases having different refractive indexes and scattering light when transmitting it. One phase of a larger refractive index includes a plenty of areas having a columnar structure extending in the thickness direction of a film and arranged to be perpendicular to a normal to the film, and exhibiting a maximum scattering angle of 10-40°.

Description

TECHNICAL FIELD [0001] The invention relates to a surface light source device, in particular a surface light source device used as a backlight in an image display device such as a liquid crystal display device or the like. BACKGROUND TECHNOLOGY [0002] Surface light source devices are conventionally used in image display devices such as liquid crystal display devices to illuminate the display panel. These surface light source devices generally comprise a light-emitting unit, a light guide, a reflector, a prism sheet and the like. A conventional surface light source device will now be described with reference to the drawings. FIG. 1 and FIG. 2 are schematic cross-sections which illustrate an example of a conventional surface light source device, and comprise light-emitting unit 1, light guide 2, reflector 3, light-diffusing film 4, and prism sheet 5. In the surface light source device of FIG. 1, the light-emitting unit comprises a light-emitting diode (LED) which is a point light sour...

AI Technical Summary

Benefits of technology

[0007] Having made a detailed study of this problem, the inventors have been able to discover that it can be resolved by positioning a light-diffusing film with directionality beside the light-outputting surface of said light guide. Directionality refers to the fact that the degree of diffusion differs with direction, and unevenness in brightness can be prevented by causing the angles (directionality) at which diffusion occurs in the light-diffusing sheet to appropriately correspond with the angles at which unevenness in brightness occurs.

Problems solved by technology

However even in this case, the difficulty remains that light output from the point light source, light-emitting diode 11, has the directionality shown in FIG. 3, and so although portions centered on the light from the central part output laterally from point light source 11 in FIG. 4(b) are bright, the four corners are dark, the upper left and bottom left corners particularly so.

However, with the surface light source device shown in FIG. 1 or the surface light source device shown in FIG. 2, the directionality of the light output from the prism pattern is as shown by the solid line in FIG. 3, and due to the characteristics of this directional light point light source, the directionality of the light output from the prism pattern is liable to cause radial patterns of unevenness in the light of the type shown by the solid curve in FIG. 3, unevenness in the brightness being particularly visible from an oblique direction.

However, increasing the number of point light sources raises the issue of increased cost.

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